230 PRINCIPLES. OF ANIMAL NUTRITION. 
sent its potential energy by 0 and accordingly that of the system 
starch + oxygen by 4183 cals. for each gram of starch. This value 
is called the heat of combustion of starch, and shows how much 
energy can be liberated from this substance by its conversion into 
CO, and H,O. It is common to speak of this as the potential energy 
of the starch, and the expression has the advantage of brevity, 
but it should not be forgotten that it is really the potential energy 
of the system C,H,,O, + 60, as compared with the system 
6CO, + 5H,0. 
In like manner the heat of combustion of any organic com- 
pound, or of any mixture of compounds such as a feeding-stuff, 
represents the amount of energy which a.given weight of it evolves 
in the form of heat when completely oxidized. In the case of 
nitrogenous bodies the final products are CO,, H,O, N,, and in 
case of proteids SO,. 
Heats of combustion may be determined at constant pressure 
or at constant volume. When the substance is burned under ordi- 
nary atmospheric pressure the amount of heat evolved may include, 
besides that due to the difference in the chemical energy of the 
substance before and after burning, a mechanical component due 
to the fact that the volume of the products is not the same as that 
of the original substances. If it is greater, work is done in 
overcoming atmospheric pressure and the heat production is 
diminished by a corresponding amount. In the contrary case, work 
is done by the atmosphere upon the products of combustion and 
heat is evolved. When the substance is burned in a confined 
volume of oxygen, as in the bomb-calorimeter, the possibility of 
such mechanical action is eliminated and we obtain a quantity of 
heat representing solely the difference in chemical energy. The heats 
of combustion at constant volume are therefore, from a theoretical 
point of view, the more correct. On the other hand, however, all 
ordinary processes of combustion, including those occurring in the 
animal organism, take place under atmospheric pressure, which is 
practically constant, and therefore the actual heat value of a sub- 
stance oxidized in the body is measured by its heat of combustion 
at constant pressure. If there is no change in volume during the 
combustion, then the two heats of combustion are, of course, iden- 
tical. This is the case, for example, with the carbohydrates, which 
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